But incredibly, Titanic had a sister ship, Britannic, which also sank to the bottom of the ocean only a few years later, after striking a mine in the Aegean Sea.

In the wake of the Titanic disaster, Britannic was re-engineered and strengthened in Belfast’s Harland and Wolff shipyard.

And yet, on this day 100 years ago – November 21, 1916 – she sank in just 55 minutes, three times faster than Titanic.

While the story of the Titanic is known all over the world, the fate of its sister ship the Britannic is one of the UK’s greatest untold maritime disasters. It remains the largest passenger ship situated on the sea floor.

After five voyages to and from the Mediterranean, Britannic was navigating into the Kea Channel on the morning of November 21, 1916 when there was a large explosion that caused her to shudder violently.

It was first thought to have been a torpedo from a U-Boat that hit Britannic, but it is more generally accepted today that it was a mine.

Britannic listed quickly and sank before Captain Bartlett was able to beach his vessel. As she was on her way to take on wounded soldiers, the ship was not at full capacity. There were just over 1,000 people on board the vessel which was capable of holding around 3,300.

Most of her crew survived, except for 30 who had taken to the lifeboats which, horrifically, were sucked into Britannic’s fast-spinning propellers.

As part of a new BBC One NI film, presenters Kate Humble and Andy Torbet piece together exactly what happened in those 55 minutes in “Titanic’s Tragic Twin: The Britannic Disaster”.

Using rarely-seen, unpublished diaries and letters from the captain and crew, the new documentary re-creates the Britannic survivors’ fight for their lives, and investigates how and why her victims died horribly – and avoidably.

While Torbet makes a dangerous dive to the Britannic wreck, Humble speaks to descendants of survivors, many of whom have been tracked down for the very first time on television.

The characters she uncovers include Violet ‘Miss Unsinkable’ Jessop, who survived both Titanic and Britannic. Nurse Jessop, born of Irish parents in Argentina, was one of the young nurses on Britannic.

Also among the fascinating people explored in the documentary are Captain ‘Iceberg Charlie’ Bartlett and Archie Jewell, who was working on Britannic’s deck when disaster struck. There is also an account of an unknown Ulsterman working below deck.

Titanic’s Tragic Twin: The Britannic Disaster, made by 360 Production, is on BBC One Northern Ireland tonight at 9pm. It will also be shown on BBC Two on Monday, December 5.

Meanwhile, special commemoration events are being hosted by Titanic Belfast in partnership with the Belfast Titanic Society to mark the 100th anniversary of the sinking of the Britannic. Some of its fittings, including her staircase and railings, will be on show in Titanic Belfast’s Atrium and a commemorative plaque will be unveiled.